ci/cd: create/use Tailscale user for hex.pm

[dylan-tailscale]

Note

It's also possible to create/use an organization (rather than a user) for Tailscale on hex.pm, but it's a paid feature and we have to figure out the mechanics of payment/approval/all that. If an organization is still preferable over a user account, update this issue with the appropriate requirements.

Create a user on hex.pm for Tailscale that can be used for publishing the Elixir tailscale package from the tailscale-rs CI/CD publish workflow.

• User must have 2FA enabled.
• Creds (including 2FA) must be saved in a credential store that only the tailscale-rs owners have access to.
• User must be an owner on the tailscale package.
• User must be able to publish new versions of the tailscale package to hex.pm.
• User must have an API key that allows publishing to hex.pm from CI/CD jobs.
• User account must be branded with Tailscale logo, etc.
• API key is added to GitHub secrets for publish workflow.
• Individual user accounts (currently Dave, Dylan, and Nathan) are removed from the tailscale package; the new Tailscale user is the only owner.

This is a mostly mechanical process; the effort is in testing with our current publishing setup.